1. Field of the Invention
The present invention relates to a technology for detecting a memory effect generated in a secondary battery such as a nickel-metal hydride secondary battery mounted on an electric vehicle (PEV), a hybrid vehicle (HEV) and the like, and resolving the memory effect.
2. Related Background Art
Secondary batteries include a lead battery, a nickel-cadmium (Nixe2x80x94Cd) battery, a nickel-metal hydride (Ni-MH) battery, a lithium-ion battery, and the like. These batteries have a characteristic of being charged by connecting to an external power source from which a predetermined current is supplied, as electrical power is consumed. With this characteristic, these batteries have been used for various equipment.
For example, the batteries are mounted on a vehicle and serve as a battery for starting an engine, which supplies electrical power to a spark plug of the engine at the time of starting the engine. Recently, Ni-MH batteries are used also for a main power source for driving a motor of an electric vehicle (PEV) and a so-called hybrid vehicle (HEV) provided with an engine and a motor.
For instance, in the case of a secondary battery mounted on the HEV, charge/discharge of the secondary battery might be repeated while the vehicle is in motion. In the HEV, when an output from an engine is larger than a power required for driving the vehicle, then surplus power is used for driving a generator so as to charge a secondary battery. Conversely, when an output from the engine is smaller, then electrical power from the secondary battery is used for driving the motor so as to compensate a shortage of the power. In the latter case, the secondary battery is discharged. Such a repetition of charge and discharge is carried out in accordance with the running state of the vehicle, the state of charge of the battery, and the operation by a driver.
In this way, if a shallow (incomplete) charge and discharge are repeated with respect to the second battery, then a voltage in the last stage of discharge is lowered, which causes decrease in the amount of discharge energy that can be used actually, so that a so-called memory effect occurs. Normally, the effective capacity of a secondary battery whose amount of discharge energy is lowered temporarily due to this memory effect can be recovered by carrying out deep (complete) discharge once. For example, in the case of a Ni-MH battery, the memory effect can be resolved by forcibly discharging it until a voltage of each cell is decreased to 1V.
In order to avoid a decrease in the amount of discharge energy due to this memory effect, generally, a refresh capability of discharging a battery completely before charge is given to a charger, or refresh discharge is carried out automatically according to the number of charges.
However, in a secondary battery system mounted on the HEV, in order to obtain an output required for driving a motor, a plurality of single cells are connected in series to make up an assembled battery, and a voltage, current, and temperature of the battery are monitored so as to repeat charge and discharge in a state of a residual battery capacity (SOC) of the secondary battery within a range of 50% to 70% of the full charged state. In such a system, complete discharge carried out when the vehicle in motion for the purpose of resolving the memory effect might lead to a significant failure of the vehicle such as stopping on the road. Therefore, it is required that after the vehicle is stopped at a charging site and complete discharge is carried out for the secondary battery, then the secondary battery is charged into a full charged state for a predetermined time period.
As stated above, in the case of the HEV, complete charge and discharge cannot be carried out while the vehicle is in motion. Therefore, it is considerably difficult to confirm the generation of the memory effect while the vehicle is in motion. In addition, since the refresh discharge and complete discharge are difficult for the HEV, it also is difficult to resolve the memory effect while the vehicle is in motion.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method of detecting and resolving the memory effect, by which generation of the memory effect can be detected with high precision and resolved even when the vehicle is in motion.
To fulfill the above-stated object, a first method for detecting a memory effect according to the present invention includes the steps of detecting a current flowing through a secondary battery configured by combining a plurality of single cells into an assembled battery; performing an operation on a variation in a residual battery capacity for a predetermined time period according to at least current integration by multiplying the detected current by a predetermined charge efficiency; detecting a temperature of the secondary battery; calculating a variation in a no-load voltage for the predetermined time period, based on the detected current and an internal resistance corresponding to the detected temperature and the residual battery capacity obtained by the operation; calculating a ratio of the variation in the no-load voltage to the variation in the residual battery capacity; and if the ratio of the variation in the no-load voltage to the variation in the residual battery capacity is not less than a predetermined threshold value, judging as a memory effect occurring in the secondary battery.
To fulfill the above-stated object, a second method for detecting a memory effect according to the present invention includes the steps of: detecting a current flowing through a secondary battery configured by combining a plurality of single cells into an assembled battery and used in a middle charge state; performing an operation on a variation in a residual battery capacity for a predetermined time period according to at least current integration by multiplying the detected current by a predetermined charge efficiency; detecting a temperature of the secondary battery; calculating a variation in a no-load voltage for the predetermined time period, based on the detected current and an internal resistance corresponding to the detected temperature and the residual battery capacity obtained by the operation; calculating a ratio of the variation in the no-load voltage to the variation in the residual battery capacity; and if the ratio of the variation in the no-load voltage to the variation in the residual battery capacity is not less than a predetermined threshold value, judging as a memory effect occurring in the secondary battery.
In the first and the second detecting methods, it is preferable that the predetermined charge efficiency is determined based on the detected temperature and the residual battery capacity obtained by the current operation.
In addition, in the first and the second detecting methods, it is preferable that the predetermined threshold value is determined based on a level of memory effect intended to be judged.
To fulfill the above-stated object, a first method for resolving a memory effect according to the present invention includes the step of, if the battery is judged to have a memory effect according to the first or the second detecting method, setting a control range of the residual battery capacity broader than a normal usage range.
To fulfill the above-stated object, a second method for resolving a memory effect according to the present invention includes the step of, if the battery is judged to have a memory effect according to the first or the second detecting method, prohibiting correction of a residual battery capacity obtained by the operation based on a voltage variation for a predetermined time period.
To fulfill the above-stated object, a third method for detecting a memory effect according to the present invention includes the steps of: detecting a current flowing through a secondary battery configured by combining a plurality of single cells into an assembled battery; detecting an output voltage from the secondary battery while associating the output voltage with the detected current; calculating a no-load voltage at the detected current being zero, based on a variation in the output voltage with respect to the detected current for a predetermined time period; performing an operation on a residual battery capacity for the predetermined time period according to at least current integration by multiplying the detected current by a predetermined charge efficiency, based on the calculated no-load voltage; calculating a correlation coefficient between the no-load voltage and the residual battery capacity obtained by the operation in a predetermined variation range; and if the correlation coefficient is not more than a predetermined threshold value, judging as a memory effect occurring in the secondary battery.
To fulfill the above-stated object, a fourth method for detecting a memory effect according to the present invention includes the steps of: detecting a current flowing through a secondary battery configured by combining a plurality of single cells into an assembled battery and used in a middle charge state; detecting an output voltage from the secondary battery while associating the output voltage with the detected current; calculating a no-load voltage at the detected current being zero, based on a variation in the output voltage with respect to the detected current for a predetermined time period; performing an operation on a residual battery capacity for the predetermined time period according to at least current integration by multiplying the detected current by a predetermined charge efficiency, based on the calculated no-load voltage; calculating a correlation coefficient between the no-load voltage and the residual battery capacity obtained by the operation in a predetermined variation range; and if the correlation coefficient is not more than a predetermined threshold value, judging as a memory effect occurring in the secondary battery.
In the third and the fourth detecting methods, it is preferable that a temperature of the secondary battery is detected, and the predetermined charge efficiency is determined based on the detected temperature and the residual battery capacity obtained by the current operation.
In the third and the fourth detecting methods, it is preferable that the calculation of the no-load voltage is carried out when a deviation of the output voltage corresponding to the detected current within predetermined ranges in a charge direction and a discharge direction is within a predetermined variance range.
Furthermore, it is preferable that the predetermined threshold value is determined based on a level of memory effect intended to be judged.
To fulfill the above-stated object, a third method for resolving a memory effect according to the present invention includes the steps of if the battery is judged to have a memory effect according to the third or the fourth memory effect detecting method, calculating an area formed by a variation curve of the no-load voltage versus a residual battery capacity in a normal usage range, and based on a difference between the area and an area formed by a variation curve of the no-load voltage versus a residual battery capacity in the normal usage range when a memory effect does not occur, setting a control range of the residual battery capacity broader than the normal usage range.
To fulfill the above-stated object, a fourth method for resolving a memory effect according to the present invention includes the steps of: if the battery is judged to have a memory effect according to the third or the fourth memory effect detecting method, calculating an area formed by a variation curve of the no-load voltage versus a residual battery capacity in a normal usage range, and based on a difference between the area and an area formed by a variation curve of the no-load voltage versus a residual battery capacity in the normal usage range when a memory effect does not occur, prohibiting correction of a residual battery capacity obtained by the operation based on a voltage variation for a predetermined time period.
According to the above-stated methods, even in the application for HEVs, the generation of the memory effect can be detected easily and accurately while the vehicle is in motion, and the generated memory effect can be resolved easily based on the degree thereof. Therefore, a high energy efficiency can be maintained in a secondary battery system.